The present invention relates generally to a method of applying an electronic material composition to an interface surface for use in connection with heat generating electronic devices. More particularly, this invention relates to a new method of applying a phase change thermal interface material to the contact surface of a heat dissipation device such as a heat sink.
In the electronics and computer industries, it has been well known to employ various types of electronic device packages and integrated circuit chips, such as the PENTIUM central processing unit chip (CPU) manufactured by Intel Corporation and RAM (random access memory) chips. These integrated circuit chips generate a great deal of heat during operation, which must be removed to prevent adverse effects on operation of the system into which the device is installed. In the prior art, heat sinks are commonly placed in thermal communication with the heat-generating device to help dissipate heat therefrom, to avoid the above-described adverse effects due to overheating. Heat sinks are often placed and maintained in direct communication with the heat-generating surface of the device to be cooled.
It is well known that the most critical locations that effect the overall performance of these heat transfer assemblies are the interface points. These locations are where two different materials mate to one another introducing two contact surfaces and often an air gap across which the heat being dissipated must be transferred. Generally, the contact surfaces are not always perfectly flat due to milling or manufacturing tolerances thus creating gaps between the heat generating surface and the heat dissipating devices thereby increasing the overall thermal resistance of the assembly. These imperfections and gaps between the mating surfaces often contain small pockets of air and thus reduce the heat transfer potential across the interface between the heat generating surface and the heat-dissipating device.
Various materials have been employed in the prior art in an attempt to bridge this interface gap. In particular, organic base materials such as polysiloxane oils or polysiloxane elastomeric rubbers and thermoplastic materials such as PVC, polypropylene, etc. loaded with thermally conducting ceramics or other fillers such as aluminum nitride, boron nitride or zinc oxide have been used to impart thermally conducting properties to the organic base material. In the case of polysiloxane oils loaded with thermally conducting materials, these materials are applied by smearing the heat sink or other electronic component with the thermally conducting paste and then securing the heat sink in place by mechanical means using clips or screws. The prior art, thermal greases show superior film forming and gap filling characteristics between uneven surfaces thus providing an intimate contact between the surface of the heat sink and the surface of the heat-generating source. However, it has been found that the use of thermal greases exhibit poor adhesions to the surfaces of the heat sink and heat generating surface, thus effectively seeping out from between the heat sink and the heat generating surface, causing air voids to form between the two surfaces, ultimately leading to hot spots. Moreover, excessive pressure placed upon the heat sink by the mechanical fasteners accelerates this seepage from between the heat sink and the surface of the heat-generating surface. It has been reported that excessive squeeze out of polysiloxane oils can evaporate and recondense on sensitive parts of the surrounding microcircuits. The recondensed oils lead to the formation of silicates thereby interfering with the function of the microprocessor and eventually causing failure.
In the case of polysiloxane rubbers and thermoplastic polymers, these materials are typically cast in sheet form and die cut into shapes corresponding to the shape of the heat sink and heat generating device. The resulting preformed sheet is then applied to the surface of the heat-generating surface securing the heat sink by means of clips or screws. The precut films solve the problems associated with greases but do not provide adequate intimate contact required for optimum heat transference between the heat generating source and the heat sink. However, the precut films, often of a phase change material, do not always wet out completely against the surface of the heat dissipation device and the added step of cutting preforms and manually applying the pad adds cost to the assembly process in addition to increasing waste and potential error, as these precut pads tend to be quite fragile. Furthermore, these types of materials show variable performance due to variation in the thickness of the pad and the amount of pressure applied to the thermally conducting precut film, based upon the mechanical device or action used to secure the heat sink.
Therefore, in view of the foregoing, heat transfer assemblies that include integral thermal transfer interfaces for mating the assembly to a heat-generating surface are highly desired. There is also a demand for a heat dissipating assembly for use in an electronic device that is lightweight, has an integral interface surface with gap bridging properties and a reduction in interface material waste when applied to heat dissipation devices having complex geometries.